Improve error message when mergability of PR is not known

According to
https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-a-pull-request,
`mergeable` may be `null` if the background checks to compute
mergability are not finished running. Improve the error message for this
edge case.

Author: hyperupcall

index.js

@@ -170,6 +170,12 @@ class Actor {
 
     const { octokit, thisRepo, issue, sender } = this;
 
+    // Don't try merge if mergability is not yet known
+    if (prInfo.data.mergeable === null) {
+      await octokit.issues.createComment({ ...thisRepo, issue_number: issue.number, body: `Sorry @${sender}, this PR is still running background checks to compute mergeability. They'll need to complete before this can be merged.` });
+      return
+    }
+
     // Don't try merge unmergable stuff
     if (!prInfo.data.mergeable) {
       await octokit.issues.createComment({ ...thisRepo, issue_number: issue.number, body: `Sorry @${sender}, this PR has merge conflicts. They'll need to be fixed before this can be merged.` });